1. Field of the Invention
The present invention relates to a receiver.
2. Description of the Related Art
A “millimeter wave passive image sensor” which detects minute millimeter wave signals irradiated from substances needs a receiver with high sensitivity.
FIG. 6 is a diagram showing a configuration example of a total-power receiver as an example of a receiver (see the following Non-Patent Document 1). A total-power receiver is frequently used since its circuit configuration is simple. The circuit configuration will be briefly described. A low noise amplifier (LNA) 104 amplifies a signal received from an antenna 102. A wave detector 105 detects the amplified signal. An integrator 111 integrates the detected signal and outputs the signal to a signal processing circuit 112. The problem of the total-power receiver is that when a signal level is small, especially when it is close to a noise level which the low noise amplifier 104 generates, the total-power receiver cannot detect a signal. When signal power is small, the sensitivity of the total-power receiver cannot be made high.
FIG. 7 is a configuration example of a Dicke type receiver which is designed for the purpose of making a total-power receiver highly sensitive (see Non-Patent Document 1). An SPDT (Single Pole Double Through) switch 702 is disposed at a pre-stage of the low noise amplifier 104, and an SPDT switch 106 is disposed at a post-stage of the detector 105. A switch driver 703 simultaneously controls the switches 106 and 702. When the switch 702 is connected to the antenna 102, the switch 106 is connected to an arithmetic unit 108. When the switch 702 is connected to a reference signal source 701, the switch 106 is connected to an arithmetic unit 109. The arithmetic unit 108 multiplies a signal received by the antenna 102 by +1. The reference signal source 701 outputs a reference signal. The arithmetic unit 109 multiplies a reference signal by −1. By controlling the switches 702 and 106, the signal received by the antenna 102 is amplified and detected for a half of a period, and the reference signal (usually, heat noise generated by resistance) is amplified and detected for the remaining half of the period. An adder 110 adds output signals of the arithmetic units 108 and 109. The integrator 111 integrates an output signal of the adder 110, and outputs the signal to the signal processing circuit 112. By subtracting the signal of the antenna 102 and the reference signal which are amplified, a noise component generated by the low noise amplifier 104 can be cancelled.
FIG. 8 is a diagram showing another configuration example of a Dicke type receiver (see the following Non-Patent Document 2). In this receiver, instead of the SPDT switch 702 of FIG. 7, a chopper 801 is disposed at the pre-stage of the antenna 102. A radio wave absorber 802 is bonded to the chopper 801. An area 803 does not absorb a radio wave. A switch driver 804 controls the rotation of the chopper 801 and switching of the switch 106. By rotating the chopper 801, it is operated as a switch.
Further, the following Non-Patent Document 3 discloses one example of the SPDT switch using a transistor. Further, the following Patent Document 4 discloses one example of the switch using a diode.    [Non-Patent Document 1] M. E. Tiuri, “Radio Astronomy Receivers”, IEEE Trans. On Antenna and Propagation, vol. AP, pp. 930-938, December 1964.    [Non-Patent Document 2] M. K. Joung, Thesis for doctoral degree, University of Tohoku, 2004    [Non-Patent Document 3] Yu-Jiu Wang et al., “A V-Band MMIC SPDT Passive HEMT Switch Using Impedance Transformation Networks”, 2001 IEEE MTT-S Int. Microwave Symp., 2001    [Non-Patent Document 4] Tanaka et al. “A 76-77 GHz High Isolation GaAs PIN-Diode Switch MMIC” R&D Review of Toyota CRDL Vol. 37 No. 2
The first problem of the above described receivers in FIGS. 7 and 8 is that they require relatively large areas. Therefore, when a number of receivers are used, reduction in size is difficult. In the case of FIG. 8, the size of the chopper 801 requires the area several times as large as the antenna 102.
FIG. 9 is a diagram showing one example of the SPDT switches 702 and 106 used in FIG. 7. The SPDT switch requires three matching circuits 901 to 903 and a distributor 906 in addition to SPST (Single Pole Single Through) switches 904 and 905. The matching circuits 901 to 903 are circuits which reduces reflection of power. The matching circuits 901 to 903 usually use transmission lines and require relatively large circuit spaces. Since the three matching circuits 901 to 903 are included, it is difficult to reduce the size of the receiver.
The second problem of the above described receivers in FIGS. 7 and 8 is that adjustment of the reference signal level is difficult. In order to change the reference signal level in the case of FIG. 8, no method is available except for the method in which the kind of the radio wave absorber 802 is changed. In the case of FIG. 7, the method in which the level of thermal noise is changed by increasing the temperature of the resistor or cooling the resistor can be considered. In the case of increasing the temperature of the resistor, influence of the temperature change simultaneously occurs to the low noise amplifier 104 or the like, and proper adjustment cannot be performed.